Are you thinking about using the potatoes in the form of slices on the pizza or in the form of mashed potatoes? When I last used potatoes on a pizza, I used the small red "new" potatoes. I sliced them into 1/8"-thick slices, and combined them with olive oil, Kosher salt, minced fresh garlic, chopped fresh oregano leaves, chopped fresh rosemary, and ground white pepper. The potato slices were then baked for about 45 minutes in a 325-degree F oven, occasionally turning them to get proper browning. The pizza itself was a "Rosemary Chicken and Potato Pizza" using a recipe from the California Pizza Kitchen Cookbook. It is quite a good pizza.